Examples of an inspection illumination device used for inspection of the appearance and the like of products include, as described in Patent Literature 1, coaxial illumination that makes an imaging direction and a direction in which an inspection object is illuminated coincide with each other. With coaxial illumination, included is: a light source that emits inspection light in a horizontal direction; and a half mirror that is arranged at an angle between the inspection object and an imaging device arranged above the inspection object; wherein the half mirror is configured so as to reflect the inspection light toward the inspection object as well as transmit reflected light from the inspection object toward the imaging device side.
Meanwhile, an ability to detect feature points such as defects from an imaged picture image that are difficult to detect even using an inspection illumination device as described above, has been demanded in recent years. More specifically, there are cases where inspection is difficult, such as a case where the shape of a product as an inspection object is special or complicated, and therefore it is difficult to apply inspection light at a sufficient intensity or with a sufficient light amount, and a case where even if inspection light can be applied, a reflected light amount from parts other than a part to be inspected is too large, or a feature point such as a defect is too small or subtle, and therefore contrast is unlikely to appear.
For example, it may be possible to increase inspection accuracy by limiting an illumination range of inspection light to only an inspection object using a diaphragm or the like, and thereby reducing stray light including reflected light and scattered light from objects other than the inspection object.
However, even if stray light entering an imaging device can be reduced by such a method, a very minute defect or the like may not produce a change in the brightness of an imaged picture image, and may not be detectable as a defect.
More specifically, even if a reflection direction of the applied inspection light is subtly changed by a minute defect or the like on an inspection object, in the case where the change is small enough to fall within an observation solid angle of an imaging device, no change is produced in the brightness of an imaged picture image, regardless of the presence or absence of a defect. Accordingly, such a minute defect or the like cannot be detected as machine vision.